"I put him at a tough table where he would be grilled and have to answer questions," said Esther Coopersmith with the delight of a stage manager whose show has been a smashing success.

And there was United Nations Ambassador Donald F. McHenry, looking as cool and collected as he does on TV, sitting amid the chairman of the House Foreign Affairs Committee, five senators and a television reporter for a network morning show.

Ambassador McHenry, who came for Sunday brunch at the Coppersmith Potomac estate after his weekly conference with Secretary of State Cyrus Vance, did have one flustered moment -- when he opened a gift to find an Ayatollah Khomeini dartboard inside.

McHenry quickly looked around and requested thay photographers take no pictures, adding in a low voice, "until we get the hostages back."

After opening the dartboard package and another gift -- a United Nations T-shirt with "No. 1" on the back -- McHenry may have wished that he had followed United Nations procedures.

"We are not supposed to open packages until they all have gone to Security for a check," he said.

The brunch in honor of McHenry was given by the Jack Coopersmiths as Mrs. Coopersmith nears the end of her term as a public member of the American delegation to the 34th U.N. General Assembly. President Carter appointed her last September.

For her U.N. boss, Mrs. Coopersmith had arranged a "tough table" -- Rep. Clement J. Zablocki (D-Wis.), head of the House Foreign Affairs Committee, and sens. Thomas Eagleton (D-Mo.), John Glenn (D-Ohio), Alan Cranston (D-Calif), Bennett JOHNSTON (D-La.) and James Sasser (D-Tenn).

Earlier, McHenry said he expected that the General Assembly would adjourn on Tuesday as scheduled.

Asked how the United States would explain on the one hand lifting sanctions on Rhodesia before United Nations action, and on the other a possible American request for U.N. sanctions against Iran, McHenry emphasized that the two situations have only a "superficial" resemblance.

The U.N. resolution on the Rhodesian sanctions, he said, specified the return of British authority, and that condition he said, has been fulfilled.

Mrs. Coopersmith -- a woman of many enthusiasms -- spoke of Ambassador McHenry as a professional diplomat "turned in" to the world and one who knows both his "institution and cast of characters." And after three months of working with him, she said, she could confirm that he is as much the "cool cat" as he has appeared on TV handling tough questions on Iran or carrying on airport negotiations after the defection of Russian ballet dancer Alexander Godunov.

McHenry who succeeded Andrew Young as U.n. ambassador, said the United Nations, as an institution, is "facing a very important test" in the Iranian crisis.

On a sun-brightened Sunday afternoon, many of the brunch guests at the Coopersmiths' had been in the midst of Christmas preparations.

Sen. Nancy Landon Kassebaum (R-Kan.) was hoping to get some Christmas shopping in after the brunch. She said she expected Congress to recess on Friday:

"They have to -- everyone has reservations to go home."

Also looking for a recess this week was Sen. Strom Thurmond (R-S.C.), who expects to go to two bowl games in which South Carolina teams will compete. And Frank Moore, White House Congressional liaison, added, "But they may be back by New Year's."

Mrs. Coopersmith, who is energetic Democratic fund-raiser credited with bringing $8 million to party coffers, had some kind words for Rep. Larry Winn, of Kansas, her Republican colleague as a public member on the U.N. delegation.

For his part, Winn said, "It's a great experience for me to be in this room," looking out over all the Democrats.

Winn told the story of how he had talked to some African delegates at the U.N. and heard their complaints about the difficulties of being in the minority.

"I told them, 'You ought to be a Republican.'"